Most electronic components, particularly solid state devices such as diodes, transistors and integrated circuitry, produce significant quantities of heat, and to maintain their reliable operation it is necessary to remove heat from the operating components. Numerous means of promoting heat dissipation from operating electronic components have been proposed in the art. The principal mode of heat transfer in many designs is conduction of generated heat to a heat sink, such as the device package and/or circuit board, which is itself cooled by convection and radiation. The effectiveness of such design depends critically on the efficiency of heat transfer between the device and the heat sink.
One of the most common means for thermally coupling heat generating chips and associated heat sinks is by application of a thermally conductive grease between the chip and the heat sink. Heat generated from the chip is efficiently conducted from the chip by the grease to, for example, a module cap, where the heat is thereafter dissipated by radiation and convection into the ambient surroundings.
Thermally conductive greases for heat transfer in electronic devices are well known in the art. Typically, they comprise a liquid carrier and a thermally conductive filler in combination with other ingredients which function to thicken the grease and remove moisture from the grease. Functionally thermal greases should exhibit high thermal conductivity, high thermal stability, and low surface tension to allow them to conform to the surface roughness and to wet heat transfer surfaces for maximizing the area of thermal contact. Further, the chemical makeup of thermal greases should be such that they are non-corrosive, electrically non-conductive and phase stable, i.e., non-bleeding and resistant to shear induced flocculation.
The liquid carriers utilized in most commercially available thermally conductive greases are mineral oils or, more commonly, silicone fluids. In combination with a thermally conductive filler, liquid silicones enable thermally conductive greases to meet each of the critical functional requirements for such products. Yet while silicone greases have generally functioned well, they have not been without disadvantage. One problem is phase separation (i.e. bleeding). Further, they are commonly known to contaminate equipment, work stations and users'clothing. That problem is exacerbated by the fact that many commercially available silicone-based thermal greases cannot be washed or removed except by the use of flammable aliphatic and aromatic hydrocarbons, or more commonly, the halogenated hydrocarbon solvents, including particularly freons. Indeed, removal of some of the commercially available silicone-based thermal greases requires the use of hot (70.degree. -80.degree. C.) polyhalogenated hydrocarbon solvents.
Because there has been significant scientific evidence of the adverse impact of halogenated hydrocarbons on the stratospheric ozone layer, there has been a nationwide, indeed a worldwide, effort to reduce emissions of such compounds by implementing control, conservation and alternative manufacturing methods. Indeed, environmentalists have demanded that use of such ozone depleting solvents be eliminated from all commercial manufacturing operations. Thus from the perspective of the electronics industry there is a significant need for development of a thermal grease which retains all of the requisite functional characteristics of the art accepted silicone-based greases including surface tension/viscosity, thermal conductivity, electrical non-conductivity, thermal stability, non-corrosiveness, and phase stability and at the same time be washable from component surface without the use of environment compromising solvents.
Accordingly, it is one object of this invention to provide a hydrophilic thermally conductive thixotropic dielectric composition for thermally coupling microelectronic components to heat sinks.
It is another object of this invention to provide a method for reducing the use of environment-compromising solvents in manufacture and rework of electronic components.
It is another object of this invention to provide a non-corrosive, thermally stable, thermally conductive dielectric grease that can be cleanly washed from all surfaces with aqueous wash solutions.
Those and related objects and advantages are obtained in accordance with this invention by utilization of a non-curing hydrophilic polymer as a liquid base for a novel, heat conductive, thixotropic dielectric.